So-called “metallocene compounds” are well known as homogeneous catalysts for olefin polymerization. Methods for polymerizing olefins using the metallocene compounds, particularly those methods for stereoregularly polymerizing α-olefins, have been the subject of numerous researches for amelioration, since the reports made by W. Kaminsky, et al. on isotactic polymerization, from the viewpoints of further enhancement of polymerization activity and improvement in stereoregularity (Non-Patent Document 1).
In an exemplary research, J. A. Ewen has reported that when propylene is polymerized in the presence of a catalyst comprising aluminoxane and a metallocene compound, which is a transition metal catalyst having a ligand of isopropylidene(cyclopentadienyl)(9-fluorene) synthesized from a ligand comprising cyclopentadienyl and fluorenyl bridged by isopropylidene, a polypropylene having a high tacticity with a syndiotactic pentad fraction of greater than 0.7 can be obtained (Non-Patent Document 2).
To improve this metallocene compound, it has been attempted to enhance the stereoregularity by replacing the fluorenyl with a 2,7-di-tert-butylfluorenyl group (Patent Document 1).
In addition to that, an attempt to enhance stereoregularity by replacing the fluorenyl with a 3,6-di-tert-butylfluorenyl group (Patent Document 2), or attempts to convert the bridging moiety which joins the cyclopentadienyl and fluorenyl (Patent Documents 3 and 4), have also been reported. Furthermore, dimethylmethylene(3-tert-butyl-5-methylcyclopentadienyl)(fluorenyl)zirconium dichloride having a methyl group introduced at the 5-position of the cyclopentadienyl ring, rather than dimethylmethylene(3-tert-butylcyclopentadienyl)(fluorenyl)zirconium dichloride, gives high molecular weight isotactic polypropylene (Patent Document 5).
However, the polymerization performance of these metallocene compounds is not sufficient. Moreover, with the catalysts of related art, it was not impossible to obtain α-olefin polymers having fairly high melting point, which is an index for stereoregularity, but polymers having high molecular weights could not be obtained. Thus, there has been a demand for production of a polymer having a fairly high melting point and a high molecular weight. Also, there has been a demand for a polymer having a higher melting point compared to the polymers of related art. Moreover, in order to enable industrialization, it is demanded to produce an α-olefin polymer having the above-described features at normal temperature or above, preferably at a high temperature exceeding normal temperature, but a catalyst which is capable of such production has not been reported heretofore.
Furthermore, even if an existing catalyst has improved polymerization performance for a specific α-olefin, the same catalyst cannot be said to be necessarily suitable for the polymerization of other α-olefins, for example, ethylene, and the catalyst had to be changed whenever the type of the polymer to be produced was varied, thus giving much inconvenience during the production.
Considering such circumstances, the inventors of the present invention have devotedly conducted research. As a result, the inventors have found that when an α-olefin such as, for example, propylene, is polymerized using a catalyst for olefin polymerization containing a specific transition metal catalyst, an α-olefin polymer having a high melting point can be obtained in a polymerization process at normal temperature, as well as in a polymerization process at a high temperature which is capable of industrialization, and also have found that even in the case of polymerizing α-olefins including ethylene as the main component under high temperature polymerization conditions, an ethylene-based polymer having a high molecular weight can be obtained with high polymerization activity, that is, the catalyst exhibits high performance in a wide range of polymerization processes, thus completing the present invention (1).
Meanwhile, a propylene-based copolymer is used in a variety of uses as a thermoplastic resin material or as a modifier for thermoplastic resin. As the polymerization catalyst used in the production of propylene-based copolymers, titanium-based catalysts and metallocene-based catalysts are known. However, in the case of using a titanium-based catalyst, there are problems such as that the composition of propylene that can be produced is limited, and the compatibility is not uniform because of wide molecular weight distribution. Furthermore, a metallocene-based catalyst shows excellent properties for copolymerization with α-olefins and enables polymerization of products with a wide range of composition, but on the other hand, there are problems such as that the molecular weight is not increased when polymerization is performed at high temperatures, and that the polymerization activity is so low that cost reduction cannot be achieved.
On the other hand, J. A. Ewen et al. found that when a catalyst comprising aluminoxane and a transition metal catalyst having a ligand of isopropylidene(cyclopentadienyl)(9-fluorene), in which cyclopentadiene and fluorene are bridged by isopropylidene, is used, a polypropylene having a high tacticity with a syndiotactic pentad fraction of greater than 0.7 can be obtained (Non-Patent Document 2).
It is also reported that a copolymer of propylene and ethylene with a high molecular weight can be obtained using a catalyst similar to the transition metal catalyst exhibiting syndiotactic polypropylene activity (Patent Document 6). However, this transition metal catalyst has low polymerization performance at high temperatures, and in particular, needs further improvement in the molecular weight.
The present inventors report that a propylene-based copolymer having a high molecular weight can be obtained using a specific transition metal catalyst (Patent Document 7). However, there still is a demand for enabling production of high molecular weight polymers under higher temperature conditions.
Therefore, the present inventors have devotedly conducted research under such circumstances, and as a result, have found that a propylene-based copolymer obtained using a specific transition metal catalyst has a high molecular weight and can be produced by polymerization at high temperatures, thus completing the present invention (2).
Meanwhile, polypropylene includes isotactic polypropylene, syndiotactic polypropylene and the like, and among these, isotactic polypropylene is used in various uses because of its low cost, and excellent rigidity, heat resistance and surface gloss.
In contrast to this, it is known that syndiotactic polypropylene can be obtained by low temperature polymerization in the presence of a catalyst comprising a vanadium compound, ether and an organoaluminum compound. The polymer obtained by this method has low syndiotacticity, so that the polymer is not considered to properly show the original syndiotactic properties.
Recently, since J. A. Ewen et al. first discovered that polypropylene having high tacticity with a syndiotactic pentad fraction larger than 0.7 can be obtained by a catalyst comprising aluminoxane and a transition metal catalyst having an asymmetric ligand (J. Am. Chem. Soc., 1988, 110, 6255-6256 (Non-Patent Document 2)), numerous successful results concerning syndiotactic polypropylene have been disclosed. For example, JP-A No. 8-67713 (Patent Document 8) discloses a method for producing syndiotactic polypropylene using a catalyst comprising rac-2,2-dimethylpropylidene(1-η5-cyclopentadienyl)(1-η5-fluorenyl)dichlorometallocene of titanium, zirconium, hafnium and vanadium, and a co-catalyst. Also, the Applicant of the present invention disclosed that a syndiotactic polypropylene satisfying particular properties is produced using a polymerization catalyst comprising 1,4-cyclohexanediylidenebis[(cyclopentadienyl-9-fluorenyl)zirconium dichloride] (JP-A No. 4-802147 (Patent Document 9)).
Syndiotactic polypropylene has very high transparency, very high surface gloss and excellent flexibility compared to conventional isotactic polypropylene. Thus, in addition to the applications known for conventional isotactic polypropylene, such as films, sheets, fibers, injection molded products and blow molded products, new applications that so far could not be applied to isotactic polypropylene are expected. However, the syndiotactic polypropylene that can be obtained by the method described in the unexamined patent application publications described above, is slow in the rate of crystallization and has low crystallization temperature, thus having a problem of poor molding processability. For example, it is difficult for syndiotactic polypropylene to be crystallized even in the pelletization step during a continuous operation, and moreover the crystallization temperature is low, so that the time required in cooling an injection molded product or an extrusion processed film or sheet is much longer than that required by isotactic polypropylene. This property impedes the production rate of molded products, and consequently leads to an increase in the energy cost. There is also a need for further improvement in the moldability as well as in the balance between the heat resistance, transparency, rigidity and strength exhibited by molded products.
Materials prepared with plastics have been used in many industrial fields where needs cannot be met by using existing propylene-based resins only. Of the industrial fields, films for food containers and medical containers are required to be excellent in the balance between high heat resistance, flexibility, low temperature impact resistance, and transparency.
Recently, a demand for retort foods has been rapidly increased at home as well as in business fields, and thus the advent of packaging materials (retort pouches) which can pack a mass of foods at once has been desired. Retort foods are usually used for storage at room temperature or in a fridge or freezer over a long period of time, and thus a film for use in the packaging material is required to have a high heat sealing strength and a good low temperature impact resistance such that a content therein does not break out of the heat sealed part of the package. In addition, since the retort sterilization treatment is carried out at high temperature of about 100 to 140° C./through autoclave after the retort pouches are filled with foods and sealed, it is necessary for the heat sealed part to maintain its heat resistance and heat sealing strength during the treatment in view of ensuring the food quality. Meanwhile, since the sterilization at higher temperature for a shorter period of time leads to enhanced working efficiency as well as improvements in retaining the quality and function of the content, further improved heat resistance of a propylene-based resin for use in a sealant layer of retort pouches or the like has been demanded (JP-A No. 09-216640 (Patent Document 910)).
For medical containers, containers made from relatively flexible and soft vinyl chloride resins and ethylene/vinyl acetate copolymer resins have been generally used. These medical bags are advantageous in that contamination from outside will not happen because they have a closed system that requires no vent needle in infusion. However, since containers made from a soft vinyl chloride resin contain additives such as a plasticizer and a stabilizer, it has been necessary to prevent the dissolution thereof. In addition, since medical bags made from an ethylene/vinyl acetate copolymer resin have poor heat resistance, the resin has to be crosslinked (JP-A Nos. 2005-053131 and 2004-244044 (Patent Documents 11 and 12)).
The Applicant of the present invention has offered the following proposals before.
JP-A No. 3-12439 (Patent Document 10) proposes a syndiotactic polypropylene resin composition comprising a substantial homopolymer of propylene in which the peak intensity of syndiotactic pentad bonding of a methyl group in the spectrum determined by 13C-NMR is greater than or equal to 0.7 of the peak intensity of methyl groups in total, and a copolymer of ethylene and propylene. The composition has high syndiotacticity, and has excellent impact resistance and transparency.
JP-A No. 7-247387 (Patent Document 11) proposes a syndiotactic polypropylene resin composition comprising 50 to 99.9 parts by weight of a resin component which is composed of 50 to 99 parts by weight of syndiotactic polypropylene and 1 to 50 parts by weight of isotactic polypropylene, and 0.1 to 50 parts by weight of a plasticizer. The composition has excellent molding processability and can result in molded products having excellent transparency and flexibility. The composition also has a rapid crystallization rate and excellent molding processability.
Furthermore, JP-A No. 8-59916 (Patent Document 12) proposes a syndiotactic polypropylene resin composition comprising 97 to 99.99% by weight of a syndiotactic polypropylene of which syndiotactic pentad fraction as measured by 13C-NMR is 0.7 or more, and 0.01 to 3% by weight of polyethylene. The composition has a rapid crystallization rate and excellent molding processability.
JP-A No. 2000-191852 (Patent Document 13) proposes a flexible transparent syndiotactic polypropylene composition comprising syndiotactic polypropylene and an amorphous propylene.α-olefin copolymer. The composition has excellent transparent, flexibility, scratch resistance and heat resistance.
JP-A No. 2000-191858 (Patent Document 14) proposes a flexible transparent syndiotactic polypropylene composition comprising syndiotactic polypropylene and a propylene.ethylene copolymer which has a substantial syndiotactic structure. The composition is described to have excellent transparency, flexibility, scratch resistance and heat resistance.
However, all of these compositions described in the publications described above are still in need of further improvement in the balance between moldability, heat resistance, transparency, impact resistance, flexibility and scratch resistance.
Furthermore, all of these compositions described in the publications described above are still in need of further improvement in the balance between moldability, heat resistance, transparency, low temperature impact resistance and flexibility.
Also, all of these compositions described in the publications described above are still in need of further improvement in the balance between moldability, heat resistance, flexibility, scratch resistance, abrasion resistance and damping properties.    [Patent Document 1] JP-A No. H4-69394    [Patent Document 2] JP-A No. 2000-212194    [Patent Document 3] JP-A No. 2004-189666    [Patent Document 4] JP-A No. 2004-189667    [Patent Document 5] JP-W No. 2001-526730    [Patent Document 6] JP-A No. H2-274703    [Patent Document 7] JP-A No. 2004-161957    [Patent Document 8] JP-A No. 1996-67713    [Patent Document 9] JP-A No. 1992-802147    [Patent Document 10] JP-A No. 09-216640    [Patent Document 11] JP-A No. 2005-053131    [Patent Document 12] JP-A No. 2004-244044    [Patent Document 13] JP-A No. H3-12439    [Patent Document 14] JP-A No. H7-247387    [Patent Document 15] JP-A No. H8-59916    [Patent Document 16] JP-A No. 2000-191852    [Patent Document 17] JP-A No. 2000-191858    [Non-Patent Document 1] Angew. Chem. Int. Ed. Engl., 24, 507 (1985)    [Non-Patent Document 2] J. Am. Chem. Soc., 110, 6255-6256 (1988)